[BUG][PYTHON] Post request arguments not properly sent

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Sponsorship to speed up the bug fix or feature request (example)

Description

The issue I have is with a generated client SDK for the Wekan project.
The generated sdk fails to work (partially) for some api endpoints.
If 'content type' of an api endpoints accepts several options, but json included, json will be chosen (by default, as default (see select_header_content_type)).
When no 'schema' is defined for the input parameters, the generated code used _form_params (instead of _body_params, which later is converted to a dict) as the aggregator of the parameters.
Then, in the request function, body is None, and post_params has a list of tuples (actually key:value). The POST flow with json content-type is reached which fully IGNORES the post_params.

openapi-generator version

version 7.21.0

OpenAPI declaration file content or url

https://raw.githubusercontent.com/wekan/wekan/refs/tags/v8.00/public/api/wekan.yml
A specific example is the /api/boards POST request (ie creating boards).

post:
      operationId: new_board
      summary: Create a board
      description: |
        This allows to create a board.

         The color has to be chosen between `belize`, `nephritis`, `pomegranate`,
         `pumpkin`, `wisteria`, `moderatepink`, `strongcyan`,
         `limegreen`, `midnight`, `dark`, `relax`, `corteza`:

         <img src="https://wekan.github.io/board-colors.png" width="40%" alt="Wekan logo" />
      tags:
        - Boards
      consumes:
        - multipart/form-data
        - application/json
      parameters:
        - name: title
          in: formData
          description: |
            the new title of the board
          type: string
          required: true
        - name: owner
          in: formData
          description: |
            "ABCDE12345" <= User ID in Wekan.
                 (Not username or email)
          type: string
          required: true
        - name: isAdmin
          in: formData
          description: |
            is the owner an admin of the board (default true)
          type: boolean
          required: false
        - name: isActive
          in: formData
          description: |
            is the board active (default true)
          type: boolean
          required: false
        - name: isNoComments
          in: formData
          description: |
            disable comments (default false)
          type: boolean
          required: false
        - name: isCommentOnly
          in: formData
          description: |
            only enable comments (default false)
          type: boolean
          required: false
        - name: isWorker
          in: formData
          description: |
            only move cards, assign himself to card and comment (default false)
          type: boolean
          required: false
        - name: permission
          in: formData
          description: |
            "private" board <== Set to "public" if you
                 want public Wekan board
          type: string
          required: false
        - name: color
          in: formData
          description: |
            the color of the board
          type: string
          required: false
      produces:
        - application/json
      security:
          - UserSecurity: []
      responses:
        '200':
          description: |-
            200 response
          schema:
            type: object
            properties:
              _id:
                type: string
              defaultSwimlaneId:
                type: string

Generation Details

Generated a python SDK for that spec via something like this:
openapi-generator-cli generate -i spec.yml -g python -o ~/wekan_sdk/ --skip-validate-spec --package-name wekan --additional-properties generateSourceCodeOnly=false,packageName=wekan

Steps to reproduce

Worth mentioning that the login WORKS because the sdk generated the code to use _body_params.
The expected result is for the new_board call not to throw a 500

# sdk_client is just a `wekan_client.ApiClient` with the proper host and configuration.
import os
import wekan_client

login_api = wekan_client.LoginApi(sdk_client)
login_response = login_api.login(
	LoginRequest(
		username=os.getenv("API_USERNAME", "test_user"),
		password=os.getenv("API_PASSWORD", "test_password"),
	)
)
title = f"Test Board {uuid.uuid4().hex[:8]}"

api_instance = wekan_client.BoardsApi(sdk_client)
response = api_instance.new_board(
	title,
	login_response.id,
	is_admin=True,
	is_active=True,
	permission="public",
	color="belize",
)

Suggest a fix

Adding

                if not body and post_params:
                    body = json.dumps(dict(post_params))

Just before https://github.com/OpenAPITools/openapi-generator/blob/e6ef8ee728c8c2b4d22bc869fc81d3cf819575a7/modules/openapi-generator/src/main/resources/python/tornado/rest.mustache#L125
will allow us to support it, WITHOUT breaking anything else.
It will look something like this:

...
if re.search('json', headers['Content-Type'], re.IGNORECASE):
    if body:
        body = json.dumps(body)
    if not body and post_params:
        body = json.dumps(dict(post_params))
    request.body = body
...

I'm willing to create the PR and follow through

Thanks in advance for the help :)

Start with modules/openapi-generator/src/main/resources/python/tornado/rest.mustache near line 125 and trace how body and post_params are handled for JSON requests. Reproduce the /api/boards new_board call from the Wekan specification, then verify that form parameters are included in the JSON request and the call no longer returns a 500.